In lighting application fields, an illuminating indicator in an illuminated switch can be used to indicate the position of the illuminated switch. FIG. 1 shows a conventional lighting circuit 100. An illuminated switch module 103 includes a switch 108, a current limiting resistor 104, and an indicator 106, e.g., a light-emitting diode (LED). The lighting circuit 100 uses an incandescent lamp 110 as a light source. The current limiting resistor 104 is coupled to the indicator 106 in series. The current limiting resistor 104 and the indicator 106 are coupled to the switch 108 in parallel. When the switch 108 is turned off, a current flows through the current limiting resistor 104, the indicator 106, and the incandescent lamp 110. Because the current is relatively small, the indicator 106 is on and the incandescent lamp 110 is off.
In recent years, more and more LED light sources are used instead of incandescent lamps. FIG. 2 shows a conventional LED driving circuit 200. Elements labeled the same as FIG. 1 have similar functions. The LED driving circuit 200 includes an illuminated switch module 103, an AC/DC converter (e.g., including a bridge rectifier 210 and a filter capacitor 212), a control circuit 214, a DC/DC converter 216, and a capacitor 218. The bridge rectifier 210 is operable for converting an AC voltage from an AC power source 102 to a rectified AC voltage. The filter capacitor 212 is coupled with the bridge rectifier 210 in parallel to filter the rectified AC voltage and to provide a first DC voltage VDC1. The control circuit 214, e.g., a chip, includes a control unit 222, a charging circuit 224, a switch 226, and a bias circuit 228. The control unit 222 is operable for generating a first control signal CTR1 to control the DC/DC converter 216 and generating a second control signal CTR2 to control the bias circuit 228. The bias circuit 228 receiving the second control signal CTR2 is operable for turning the switch 226 on and off. The DC/DC converter 216 controlled by the first control signal CTR1 converts the first DC voltage VDC1 to a second DC voltage VDC2 which is supplied to an LED string 220 and a third DC voltage VDC3 which is supplied to the control circuit 214 and the capacitor 218.
When the switch 108 is turned on, initially the DC/DC converter 216 is disabled and the capacitor 218 is charged by a current from the capacitor 212 and from the power source 102 via the charging circuit 224. A voltage VDD of the capacitor 218 is applied to the control circuit 214. When the voltage VDD of the capacitor 218 increases to a threshold voltage VDDON, the control unit 222 is enabled. The control unit 222 generates the first control signal CTR1 to enable the DC/DC converter 216 and generates the second control signal CTR2 to turn off the switch 226. Since the DC/DC converter 216 is enabled, the capacitor 218 is charged by the DC/DC converter 216, e.g., a transformer.
When the switch 108 is off, a current flows through the current limiting resistor 104, the indicator 106, and the bridge rectifier 210. The current through the indicator 106 is relatively small because the resistor 104 has a relatively large resistance. The gate voltage of the switch 226 increases as the voltage across the capacitor 212 increases. When the gate voltage of the switch 226 increases to the turn-on threshold, the switch 226 is turned on by the bias circuit 228, and thus a current from the resistor 104 and the capacitor 212 charges the capacitor 218 through the switch 226 and the charging circuit 224. The voltage across the capacitor 218 increases accordingly. When the voltage VDD is greater than the threshold voltage VDDON, the control unit 222 is enabled. The control unit 222 generates the first control signal CTR1 to enable the DC/DC converter 216 and generates the second control signal CTR2 to turn off the switch 226. Thus, the DC/DC converter 216 generates the second DC voltage VDC2 to the LED string 220 and the third DC voltage VDC3 to the capacitor 218 and the control circuit 214. The LED string 220 is powered on.
The voltage across the capacitor 218 and the capacitor 212 decreases due to a power consumption of the control circuit 214 and the LED string 220. When the voltage VDD is less than a voltage VDDOFF, the control unit 222 is disabled, and thus the control unit 222 stops generating the first control signal CTR1 and the second control signal CTR2. Therefore, the DC/DC converter 216 is disabled—it stops supplying the second DC voltage VDC2 to the LED string 220. The LED string 220 is powered off. Then, the next cycle begins—the capacitor 212 is charged again, the switch 226 is turned on again, the control unit 222 is enabled again. Consequently, when the switch 108 is off, the control unit 222 is enabled periodically and the LED string 220 is powered on periodically, which causes undesired flashes.